I bet the cost of sharing a song is much less. Let's say you have a new song that a bunch of kids like, what do they do?
They check p2p programs to see if they can find it, if they don't find it that way (or they skip that step) they check torrent sites for the album, finally after they have exhausted options the weigh cost/need whether or not to buy it on itunes or get the cd.
After judging the cost I bet less than half or even less than a quarter actually buy the song, they will just move on to the next great thing and leave it at that.
There is another interesting subset that itunes has brought out though and that is the group of people that check itunes first and pay for their music... something that people should really pay attention to.
